Character-transfer ribbons can be used on typewriters and printers, e.g. for word-processing machinery or computers, utilizing a variety of devices for transferring an image of a character to a substrate, e.g. a sheet of paper, displaced on a platen, the substrate being interposed between the platen and the ribbon.
The mechanism utilized to transfer color from the ribbon to the substrate can include relatively low velocity devices such as levers which are provided with typefaces at free ends thereof, and type balls or drums which are rotated and angularly or axially shifted to dispose a selected typeface of a font thereof formed on the ball, in the striking position, the ball being then impelled against the ribbon to impact the latter against the substrate.
High velocity character-transfer devices are commonly in use in typewriters and printers for independent use or for use as part of computers, word processing equipment or electronic data output terminals.
Such print wheels (daisy wheels) generally comprise a multiplicity of radially extending deflectable arms formed at their free ends with the respective character or typeface, and provided with a common hub by which the wheel is mounted for rapid angular displacement on the wheel carrier of the machine. A hammer aligned with the free end of the arms as they are moved into the strike position is capable of imparting an impact to the respective free ends to drive the latter against the ribbon and the substrate. Such systems are of the high velocity type, i.e. the velocity with which the typeface is carried towards and resiliently withdrawn from the substrate is substantially greater than the velocity with which the typefaces are moved toward and away from the substrate with lever-type and ball-type systems.
The most common ribbon utilized in the past generation for typewriters and the like was the inked cloth ribbon, which consists generally of a woven band impregnated with ink and from which ink in the pattern of the typeface is carried onto the substrate to produce the image or the type pattern thereon. Such ribbons or cartridges or reels usually are moved back and forth past the striking position numerous times until their image-producing capacities fade, whereupon the ribbons are discarded and replaced. For this purpose the machine is equipped generally with a reversing mechanism which is effective upon each passage of the full length of ribbon to switch the direction of ribbon track.
In more recent years, a large part of the ribbon market has been taken over by so-called single-use carbon ribbon, a ribbon which consists of a synthetic resin foil carrier and a pigmented layer which is transferred to the substrate upon impact of the typeface with the back of the ribbon.
Such ribbons are comparatively expensive because they can be used only once.
It has already been proposed to provide an overlapping-impact ribbon, i.e. a ribbon in which each impact of the typeface is executed partly on fresh ribbon and partly on ribbon which has been subjected to a previous color-transfer impact. Such ribbons are also called overstrike ribbons since a portion of each typing strike is effected over regions which have been previously struck. Overstrike ribbons are provided mainly for type wheel printers. In operation, for example, the ribbon may be advanced by, say 1/3 of the character width with each character strike, so that the next strike will take place over a region corresponding to 1/3 of the width and constituted by fresh ribbon which has not previously transferred color to the substrate. The other 2/3 of the strike will be effected over previously struck ribbon.
Consequently, only part of each typing impact is effected on fresh ribbon, while the larger proportion of each impact is effected on previously impacted portions which themselves have received two or more impacts.
In the case of a 1/3 displacement in the manner described, each area of the ribbon position at the striking location receives the equivalent of 3.3 impacts and thus the entire ribbon is effectively used 3.3 times, i.e. is a 3.3 overstrike ribbon.
The advantage of this arrangement is that the ribbon becomes a multiuse ribbon in spite of the fact that it is only moved substantially unidirectionally through the typing station and does not have to be reversed. In fact, a typewriter or printer equipped with an overstrike ribbon which allows three or more strikes at each location can eliminate the ribbon return mechanism entirely, since the ribbon can be discarded economically after only a single pass.
Unfortunately, overstrike ribbons as, for example, described in German open application No. 28 23 382, comprise a thin support foil and a color-transfer coating in the form of a synthetic resin binder matrix and a coloring paste dispersed in this matrix.
The coloring paste itself consists essentially of a coloring agent or pigment, a wetting agent and fillers, all in an oil base which is immiscible with the synthetic resin of the matrix.
The oil base generally is a thioxotropic high viscosity mixture (viscosity of at least 100,000 mPa.s at 20.degree. C.) of liquid oils such as ricinus, neat's foot oil, peanut oil, glycerintrioleate, or mineral oil, and semisolid waxes such as lanolin, petrolatum etc.
To form the color-transfer coating, a solution of the binder in the solvent, which can also solubilize the oil base, is formed and applied to the carrier foil. The solvent-borne coating is then dried by vaporization of the solvent and the oil-phase is dispersed in numerous microdroplets within the binder. These droplets, which ideally intercommunicate with one another, are embedded in the binder when the latter hardens or sets.
The result is a spongy synthetic resin matrix upon a carrier foil, which is filled with the coloring paste and from which a portion of the coloring paste can be pressed (extruded) onto the substate with each strike of the typeface.
Conventional overstrike ribbons of this type have a 3.3 overstrike capacity and efforts to extend their use to a 5-fold overstrike capacity, i.e. a system in which the advance of the ribbon is reduced to 1/5 the character width for each stroke, have not been successful, since portions of the character are not effectively printed.
This is especially the case for high-velocity impact printing machines utilizing print wheels as described above.
By comparison with print balls and lever-borne typefaces which operate at a comparatively low velocity so that there is time for the coloring paste to be extruded from the binder matrix onto the substrate, high-velocity impact printing appears to bring a ribbon into contact with the paper and the typeface for too short a period to permit effective transfer.
The obvious solution, namely, a reduction in the viscosity of the coloring paste, has not succeeded in solving the problems with such ribbons.
For one thing, reduced viscosity is generally brought about by the addition of liquid oil, which dilutes the coloring matter and makes the imparted coloration somewhat pale. When attempts are made to compensate for the reduction of the viscosity by addition of oil-soluble coloring matter, the highly colored thin oil base tends to bleed readily in the fibrous paper of the substrate and to distort the image by widening the latter and rendering it duffuse and less sharp than is desired. Furthermore, the more fluid "ink" tends to penetrate the paper readily and the image can show through on the reverse side.
Another disadvantage with the low viscosity paste is the less effective coating which can be carried out because phase-separation from the binder appears to occur before the solvent has sufficiently vaporized to ensure an effective dispersion of the coloring material within the binder matrix.
Finally, when the coloring material is too thin, i.e. is of reduced viscosity, during the initial strike excessive coloring matter is transferred to the substrate so that insufficient coloring matter may remain for effective subsequent strikes. The transferred print is thus nonuniform and the problem which reducing the viscosity attempted to solve is recreated.